This invention relates to a power cable having a semiconductive shield and moisture cured insulation.
A typical electric power cable generally comprises one or more conductors in a cable core that is surrounded by several layers of polymeric materials including a first (internal) semiconductive shield layer (conductor or strand shield), an insulating layer, a second semiconductive shield layer (insulation shield or external semiconductive layer), a metallic tape or wire shield, and a protective jacket. The external semiconductive shield can be either bonded to the insulation or strippable, with most applications using strippable shields. Additional layers within this construction such as moisture impervious materials are often incorporated.
Polymeric semiconductive shields have been utilized in multilayered power cable construction for many decades. Generally, they are used to fabricate solid dielectric power cables rated for voltages greater than 1 kilo Volt (kV). These shields are used to provide layers of intermediate conductivity between the high potential conductor and the primary insulation, and between the primary insulation and the ground or neutral potential. The volume resistivity of these semiconductive materials is typically in the range of 10xe2x88x921 to 108 ohm-centimeters when measured on a completed power cable construction using the methods described in ICEA S-66-524, section 6.12, or IEC 60502-2 (1997), Annex C. Typical strippable internal or external shield compositions contain a polyolefin, such as ethylene/vinyl acetate copolymer with a high vinyl acetate content, conductive carbon black, an organic peroxide crosslinking agent, and other conventional additives such as a nitrile rubber, which functions as a strip force reduction aid, processing aids, and antioxidants. These compositions are usually prepared in granular or pellet form. Polyolefin formulations such as these are disclosed in U.S. Pat. No. 4,286,023 and European Patent Application 420 271. The shield composition is, typically, introduced into an extruder where it is co-extruded around an electrical conductor at a temperature lower than the decomposition temperature of the organic peroxide to form a cable. The cable is then exposed to higher temperatures at which the organic peroxide decomposes to provide free radicals, which crosslink the polymer. The electrical conductor can be, for example, made of annealed copper, semihard drawn copper, hard drawn copper, or aluminum.
Polyethylenes, which are typically used as the polymeric component in the insulation layer, can be made moisture curable by making the resin hydrolyzable, which is accomplished by adding hydrolyzable groups such as xe2x80x94Si(OR)3 wherein R is a hydrocarbyl radical to the resin structure through conventional copolymerization or grafting techniques. Grafting can be effected at 210 to 250 degrees C. Suitable crosslinking agents are organic peroxides such as dicumyl peroxide; 2,5-dimethyl-2,5-di(t-butylperoxy)hexane; t-butyl cumyl peroxide; and 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3. Dicumyl peroxide is preferred. The amount of organic peroxide used in the grafting process can be in the range of 0.01 to 4 parts by weight per 100 parts by weight of the base resin.
Suitable alkoxysilane compounds, which can be used to provide the hydrolyzable group can be represented by the following formula: RRxe2x80x2SiY2 wherein R is an aliphatic unsaturated hydrocarbon group or a hydrocarbonoxy group, Rxe2x80x2 is a hydrogen atom or a saturated monovalent hydrocarbon group, and Y is an alkoxy group. Examples of R are vinyl, allyl, butenyl, cyclohexenyl, and cyclopentadienyl. The vinyl group is preferred. Examples of Y are ethoxy, methoxy, and butoxy.
Examples of ethylenically unsaturated alkoxysilanes are vinyl triethoxysilane, vinyl trimethoxysilane, and gamma-methacryloxypropyltrimethoxy-silane. The amount of alkoxysilane compound that can be used is preferably about 0.5 to about 20 parts by weight per 100 parts by weight of base resin.
Hydrolyzable groups can be added, for example, by copolymerizing ethylene with an ethylenically unsaturated compound having one or more xe2x80x94Si(OR)3 groups or grafting these silane compounds to the resin in the presence of the aforementioned organic peroxides. The hydrolyzable resins are then crosslinked by moisture, e.g., steam or hot water, in the presence of a silanol condensation catalvst such as dibutyltin dilaurate, dioctyltin maleate, dibutyltin diacetate, stannous acetate, lead naphthenate, and zinc caprylate. Dibutyltin dilaurate is preferred. The amount of silanol condensation catalyst can be in the range of about 0.001 to about 20 parts by weight per 100 parts by weight of base resin, and is preferably about 0.005 to about 5 parts by weight.
Examples of hydrolyzable copolymers and hydrolyzable grafted copolymers are ethylene/vinyltrimethoxy silane copolymer, ethylene/gamma-methacryloxypropyltrimethoxy silane copolymer, vinyltrimethoxy silane grafted ethylene/ethyl acrylate copolymer, vinyltrimethoxy silane grafted linear low density ethylene/1-butene copolymer, and vinyltrimethoxy silane grafted low density polyethylene.
In applications where moisture cured insulation is used, it is desirable to provide a moisture cured strippable semiconductive shield to protect the insulation. The shield composition would then be prepared in the same manner as the moisture cured insulation as outlined above. Unfortunately, shield compositions, which could be moisture cured, were found to have a tendency to scorch, i.e., to prematurely crosslink during extrusion. In addition to solving the scorch problem, the shield had to be easily strippable by hand or with the aid of an appropriate tool.
Further, the use of a peroxide crosslinkable insulation shield over a moisture curable insulation is not considered viable because of the incompatibility of the processing requirements for each. Typically, the peroxide system utilizes higher operating temperatures during the cure cycle, and these high temperatures interfere with the dimensional stability of the xe2x80x9cuncuredxe2x80x9d moisture curable insulation. The upshot is that the peroxide system requires a pressurized curing tube, which is an integral part of the extrusion process, while the moisture curable insulation is cured in a post extrusion stage. It was also found that while crosslinking via a peroxide did improve scorch, it did not enhance strippability.
It is apparent, then, that both the peroxide system and the moisture cure system for the insulation shield each have their drawbacks. Further, it is found especially desirable that the insulation shield have the following characteristics:
(1) a volume specific resistance of 100 ohm-centimeters or less to prevent corona degradation caused by partial delamination and gap formation;
(2) an elongation of 100 percent or more to maintain elasticity, and prevent partial delamination and gap formation when the power cable is bent or is exposed in the heat cycle;
(3) a smooth interface between the moisture cured insulation layer and the insulation shield with an absence of micro-protrusions;
(4) capable of being extruded high temperatures similar to the temperatures used for the moisture cured insulation layer, i.e., 210 to 250 degrees C.;
(5) a tensile strength of 10 Mpa or more so that the insulation shield will not be cut during stripping;
(6) a cold temperature resistance;
(7) a peel strength of 4 kilograms per 0.5 inch or less to provide for easy stripping from the moisture cured insulation layer; and
(8) a heat deformation at 120 degrees C. of 40 percent or less.
An object of this invention, therefore, is to provide a composition useful for an insulation shield for a moisture cured insulation layer, which has the above characteristics and avoids the drawbacks of peroxide and moisture cured shields. Other objects and advantages will become apparent hereinafter.
According to the invention, such a composition has been discovered. The composition comprises:
(a) one or more copolymers selected from the group consisting of (I) a copolymer of ethylene and vinyl acetate containing about 10 to about 50 percent by weight vinyl acetate and having a melt mass flow rate of about 1 to about 100 grams per 10 minutes; (II) a copolymer of ethylene and ethyl acrylate containing about 10 to about 50 percent by weight ethyl acrylate and having a melt mass flow rate of about 1 to about 100 grams per 10 minutes; and (III) a copolymer of ethylene and butyl acrylate containing about 10 to about 50 percent by weight butyl acrylate and having a melt mass flow rate of about 1 to about 100 grams per 10 minutes, and based upon 100 parts by weight of component (a):
(b) about 55 to about 200 parts by weight of a linear copolymer of ethylene and an alpha-olefin having 3 to 12 carbon atoms, the copolymer having a melt mass flow rate of about 0.1 to about 30 grams per 10 minutes and a density of 0.870 to 0.944 gram per cubic centimeter;
(c) about 5 to about 50 parts by weight of polypropylene having a melt mass flow rate of about 0.5 to about 30 grams per 10 minutes and a density of 0.900 to 0.920 gram per cubic centimeter;
(d) about 2 to about 50 parts by weight of an organopolysiloxane having the following formula: R1xR2ySiO(4xe2x88x92axe2x88x92b)/2
wherein R1 is an aliphatic unsaturated hydrocarbon group; R2 is an unsubstituted or substituted monovalent hydrocarbon group excluding aliphatic unsaturated hydrocarbon groups; x is equal to or greater than 0 but less than 1; y is greater than 0.5 but less than 3; x+y is greater than 1 but less than 3; a is greater than 0 but equal to or less than 1; and b is equal to or greater than 0.5 but equal to or less than 3;
(e) about 10 to about 350 parts by weight of carbon black; and
(f) optionally, up to about 2 parts by weight of an organic peroxide.
The resins most commonly used in semiconductive shields are elastomers of varying degrees of crystallinity from amorphous through low and medium crystallinity. These elastomers are typically ethylene/unsaturated ester copolymers, which are usually made by conventional high pressure free radical processes generally run at pressures above 15,000 psi (pounds per square inch). The ethylene/unsaturated ester copolymers used in this invention are set forth in component (a), above, i.e., one or more copolymers selected from the group consisting of (I) a copolymer of ethylene and vinyl acetate containing about 10 to about 50 percent, preferably about 15 to about 40 percent, by weight vinyl acetate and having a melt mass flow rate of about 1 to about 100 grams per 10 minutes; (II) a copolymer of ethylene and ethyl acrylate containing about 10 to about 50 percent, preferably about 15 to about 40 percent, by weight ethyl acrylate and having a melt mass flow rate of about 1 to about 100 grams per 10 minutes; and (III) a copolymer of ethylene and butyl acrylate containing about 10 to about 50 percent, preferably about 15 to about 40 percent, by weight butyl acrylate and having a melt mass flow rate of about 1 to about 100 grams per 10 minutes. The percent by weight is based on the total weight of the copolymer. Melt mass flow rate is determined under JIS (Japanese Industrial Standard) K-6760. It is measured at 190xc2x0 C. and 21.6 kilograms in grams per 10 minutes.
Component (b) is about 55 to about 200, preferably about 75 to about 150, parts by weight of a linear copolymer of ethylene and an alpha-olefin having 3 to 12 carbon atoms, the copolymer having a melt mass flow rate of about 0.1 to about 30 grams per 10 minutes and a density of 0.870 to 0.944 gram per cubic centimeter. The ethylene polymers useful in subject invention are preferably produced in the gas phase. They can also be produced in the liquid phase in solutions or slurries by conventional techniques. They are usually produced by low pressure processes, which are typically run at pressures below 1000 psi. Typical catalyst systems, which can be used to prepare these polymers are magnesium/titanium based catalyst systems, which can be exemplified by the catalyst system described in U.S. Pat. No. 4,302,565; vanadium based catalyst systems such as those described in U.S. Pat. Nos. 4,508,842 and 5,332,793; 5,342,907; and 5,410,003; a chromium based catalyst system such as that described in U.S. Pat. No. 4,101,445; a metallocene catalyst system such as that described in U.S. Pat. Nos. 4,937,299 and 5,317,036; or other transition metal catalyst systems. Many of these catalyst systems are often referred to as Ziegler-Natta catalyst systems. Catalyst systems, which use chromium or molybdenum oxides on silica-alumina supports, are also useful. Typical processes for preparing the polymers are also described in the aforementioned patents. Blends of these copolymers can be used if desired. Typical in situ polymer blends and processes and catalyst systems for providing same are described in U.S. Pat. Nos. 5,371,145 and 5,405,901. The linear copolymers can be, among others, LLDPE or VLDPE.
The linear low density polyethylene (LLDPE) can have a density in the range of 0.916 to 0.925 gram per cubic centimeter. It can be a copolymer of ethylene and one or more alpha-olefins having 3 to 12 carbon atoms, and preferably 3 to 8 carbon atoms. The preferred alpha-olefins can be exemplified by propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene, and the catalysts and processes can be the same as those mentioned above subject to variations necessary to obtain the desired densities and melt indices.
The very low density polyethylene (VLDPE) can also be a copolymer of ethylene and one or more alpha-olefins having 3 to 12 carbon atoms and preferably 3 to 8 carbon atoms. Preferred alpha-olefins are mentioned above. The density of the VLDPE can be in the range of 0.870 to 0.915 gram per cubic centimeter. It can be produced using the catalysts and processes mentioned above. The portion of the VLDPE attributed to the comonomer(s), other than ethylene, can be in the range of about 1 to about 49 percent by weight based on the weight of the copolymer and is preferably in the range of about 15 to about 40 percent by weight. A third comonomer can be included, e.g., another alpha-olefin or a diene such as ethylidene norbornene, butadiene, 1,4-hexadiene, or a dicyclopentadiene. The third comonomer can be present in an amount of about 1 to 15 percent by weight based on the weight of the copolymer and is preferably present in an amount of about 1 to about 10 percent by weight. It is preferred that the copolymer contain two or three comonomers inclusive of ethylene.
Component (c) is about 5 to about 50, preferably about 15 to about 30, parts by weight of polypropylene having a melt mass flow rate of about 0.5 to about 30 grams per 10 minutes and a density of 0.900 to 0.920 gram per cubic centimeter. The polypropylene can be a homopolymer of propylene or a copolymer of propylene and an alpha-olefin having 2 or 4 to 12 carbon atoms wherein the portion of the copolymer based on propylene is at least about 60 percent by weight based on the weight of the copolymer. Examples of the alpha-olefins are mentioned above. The polypropylene can be prepared by conventional processes such as the process described in U.S. Pat. No. 4,414,132.
Component (d) is about 2 to about 50, preferably about 2 to about 10, parts by weight of an organopolysiloxane having the following formula: R1xR2ySiO(4xe2x88x92axe2x88x92b)/2
wherein R1 is an aliphatic unsaturated hydrocarbon group; R2 is an unsubstituted or substituted monovalent hydrocarbon group excluding aliphatic unsaturated hydrocarbon groups; x is equal to or greater than 0 but less than 1; y is greater than 0.5 but less than 3; x+y is greater than 1 but less than 3; a is greater than 0 but equal to or less than 1; and b is equal to or greater than 0.5 but equal to or less than 3.
In the organopolysiloxane expressed by the aforementioned formula, R1 can be, for example, a vinyl, allyl, acryl, or methacryl group, and R2 can be an alkyl group such as methyl, ethyl, or propyl; an aryl group such as phenyl or tolyl; a cycloalkyl group such as cyclohexyl or cyclobutyl. The R2 groups can be substituted with various substituents such as halogen atoms, or cyano or mercapto.
Organopolysiloxanes, which can be used in the present invention, can be exemplified by linear, branched, cyclic, network, or stereo network structures provided that the molecular structure is within the formula, but the linear structure is preferable. The degree of polymerization of the organopolysiloxane is not particularly limited, but it preferably has a degree of polymerization which does not inhibit kneading with the ethylene copolymer.
One organopolysiloxane, which can be used is silicone gum. Another linear organopolysiloxane can be represented by the following formula: R3xe2x80x94Sixe2x80x94Oxe2x80x94(R2xe2x80x94Sixe2x80x94O)nxe2x80x94R3 wherein R is a substituted or unsubstituted monovalent hydrocarbon and n is at least 10. This compound is generally referred to as a silicone oil. R can be an alkyl or aryl group and hydrogen. Examples of the alkyl group are methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, and t-butyl. An example of the aryl group is phenyl. The Rs can be the same or different except that the Rs cannot all be hydrogen, and part of the R can be, for example, vinyl or hydroxyl; n can be 10 to 10,000 and is preferably 100 to 1000. It is desirable that the viscosity of the organopolysiloxane in liquid form is a least about 10 centistokes, and is preferably about 1000 to about 1,000,000 centistokes, at 23 degrees C.
Component (e): In order to provide a semiconductive shield it is necessary to incorporate conductive particles into the composition. These conductive particles are generally provided by particulate carbon black, which is referred to above. Useful carbon blacks can have a surface area of about 50 to about 1000 square meters per gram. The surface area is determined under ASTM D 4820-93a (Multipoint B.E.T. Nitrogen Adsorption). The carbon black can be used in the semiconductive shield composition in an amount of about 10 to about 350 parts by weight, and preferably about 40 to about 300 parts by weight. An objective is to keep the volume specific resistance at less than about 100 ohm-centimeters. Both standard conductivity and high conductivity carbon blacks can be used with standard conductivity blacks being preferred. Examples of conductive carbon blacks are the grades described by ASTM N550, N472, N351, N110, acetylene black, furnace black, and Ketjen black. The Ketjen black is particularly desirable as one third to one half the amount of Ketjen black provides the same level of conductivity as the full amount of a conventional carbon black.
Optionally, the following copolymer can be included in semiconductive shield compositions: a copolymer of acrylonitrile and butadiene wherein the acrylonitrile is present in an amount of about 30 to about 60 percent by weight based on the weight of the copolymer, and is preferably present in an amount of about 40 to about 50 percent by weight. This copolymer is also known as a nitrile rubber or an acrylonitrile/butadiene copolymer rubber. The density can be, for example, 0.98 gram per cubic centimeter and the Mooney Viscosity can be (ML 1+4) 50. A silicone rubber can be substituted for this copolymer
(f) Organic peroxide component. As noted, this component is optional, but it is preferred that it be in the insulation shield composition. The organic peroxide has an (Oxe2x80x94O) bond in the molecule, and it is preferable that it has a 10 minute half life at 100 to 220 degrees C. It assists the filling and dispersing properties of the carbon black by grafting components (a) through (d), and not initiating a crosslinking reaction with respect to these components. Examples of suitable organic peroxides follow (the figure in parenthesis is the decomposition temperature of the organic peroxide in degrees C.): succinic acid peroxide (110), benzoyl peroxide (110), t-butylperoxy-2-ethylhexanoate (113), p-chlorobenzoyl peroxide (115), t-butylperoxyisobutyrate (115), t-butylperoxyisopropylcarbonate (135), t-butylperoxylaurate (140), 2,5-dimethyl-2,5-di(benzoylperoxy)hexane (140), t-butylperoxyacetate (140), di-tbutyldiperoxyphthalate (140), t-butylperoxybenzoate (145), dicumyl peroxide (150), 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (155), tbutylcumyl peroxide (155), t-butylhydroperoxide (158), di-t-butyl peroxide (160), 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3 (170), di-isopropylbenzenehydroperoxide (170), p-menthanehydroperoxide (180), 2,5-dimethylhexane-2,5-dihydroperoxide (213), and cumenehydroperoxide (149). Among these, cumyl peroxide, 2,5-dimethyl-2,5-(tbutylperoxy)hexane, and cumenehydroperoxide are preferred.
The blending amount of the organic peroxide is up to about 2 parts by weight, and is preferably in the range of about 0.15 to about 0.8 part by weight, and more preferably in the range of about 0.3 to about 0.6 part by weight. Its function is to initiate a graft reaction between components (a) through (d), particularly components (a) and (d).
The insulation shield composition can be prepared in the following ways:
(i) Component (d) can be grafted to component (a) by kneading while heating at about 220 degrees C. Then all of the components can be fed into an extruder.
(ii) Component (d) can be grafted to component (a) by heating at about 160 degrees C. in the presence of an organic peroxide. Then all of the components can be fed into an extruder.
(iii) The polymers can be grafted to one another by kneading components (a), (b), (c), (d) and (f) together while heating at about 165 degrees C.
Conventional additives, which can be introduced into the composition, are exemplified by antioxidants, coupling agents, ultraviolet absorbers or stabilizers, antistatic agents, pigments, dyes, nucleating agents, reinforcing fillers or polymer additives, slip agents, plasticizers, processing aids, lubricants, viscosity control agents, tackifiers, anti-blocking agents, surfactants, extender oils, metal deactivators, voltage stabilizers, flame retardant fillers and additives, crosslinking agents, boosters, and catalysts, and smoke suppressants. Additives and fillers can be added in amounts ranging from less than about 0.1 to more than about 50 percent by weight based on the weight of the composition.
Examples of antioxidants are: hindered phenols such as tetrakis [methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)] methane, bis [(beta-(3,5-ditert-butyl-4-hydroxybenzyl)methylcarboxyethyl)] sulphide, 4,4xe2x80x2-thiobis(2-methyl-6-tert-butylphenol), 4,4xe2x80x2-thiobis(2-tert-butyl-5-methylphenol), 2,2xe2x80x2-thiobis(4-methyl-6-tert-butylphenol), and thiodiethylene bis(3,5-di-tert-butyl-4-hydroxy)hydrocinnamate; phosphites and phosphonites such as tris(2,4-di-tert-butylphenyl)phosphite and di-tert-butylphenyl-phosphonite; thio compounds such as dilaurylthiodipropionate, dimyristylthiodipropionate, and distearylthiodipropionate; various siloxanes; and various amines such as polymerized 2,2,4-trimethyl-1,2-dihydroquinoline, 4,4xe2x80x2-bis(alpha,alpha-demthylbenzyl)diphenylamine, and alkylated diphenylamines. Antioxidants can be used in amounts of about 0.001 to about 5 percent by weight based on the weight of the composition.
Compounding can be effected in a conventional melt/mixer or in a conventional extruder, and these terms are used in this specification interchangeably. Generally, the conductive shield composition is prepared in a melt/mixer and then pelletized using a pelletizer attachment or an extruder adapted for pelletizing. Both the melt/mixer, as the name implies, and the extruder, in effect, have melting and mixing zones although the various sections of each are known to those skilled in the art by different names. The semiconductive shield composition of the invention can be prepared in various types of melt/mixers and extruders such as a Brabender(trademark) mixer, Banbury(trademark) mixer, a roll mill, a Buss(trademark) co-kneader, a biaxial screw kneading extruder, and single or twin screw extruders. A description of a conventional extruder can be found in U.S. Pat. No. 4,857,600. In addition to melt/mixing, the extruder can coat a wire or a core of wires. An example of co-extrusion and an extruder therefor can be found in U.S. Pat. No. 5,575,965. A typical extruder has a hopper at its upstream end and a die at its downstream end. The hopper feeds into a barrel, which contains a screw. At the downstream end, between the end of the screw and the die, is a screen pack and a breaker plate. The screw portion of the extruder is considered to be divided up into three sections, the feed section, the compression section, and the metering section, and two zones, the back heat zone and the front heat zone, the sections and zones running from upstream to downstream. In the alternative, there can be multiple heating zones (more than two) along the axis running from upstream to downstream. If it has more than one barrel, the barrels are connected in series. The length to diameter ratio of each barrel is in the range of about 15:1 to about 30:1. In wire coating, where the material is crosslinked after extrusion, the die of the crosshead feeds directly into a heating zone, and this zone can be maintained at a temperature in the range of about 130xc2x0 C. to about 260xc2x0 C., and preferably in the range of about 170xc2x0 C. to about 220xc2x0 C. Double layer simultaneous extruding machines and triple layer simultaneous extruding machines are advantageously used to prepare the power cable with the various layers described above.
The advantages of the invention are excellent semiconductivity, strippability, tensile strength, processability, surface smoothness, cold temperature resistance, and heat endurance The term xe2x80x9csurroundedxe2x80x9d as it applies to a substrate being surrounded by an insulating composition, jacketing material, or other cable layer is considered to include extruding around the substrate; coating the substrate; or wrapping around the substrate as is well known by those skilled in the art. The substrate can include, for example, a core including a conductor or a bundle of conductors, or various underlying cable layers as noted above.
All molecular weights mentioned in this specification are weight average molecular weights unless otherwise designated.
The patents mentioned in this specification are incorporated by reference herein.
The invention is illustrated by the following examples